The proposed project is directed towards: 1) study of serum concentration T3, T4 and reverse T3 concentrations in health and in thyroid disease. 2) To study the sources of T3 and reverse T3 present in human circulation. 3) To study the tissue binding of thyroid hormone. 4) To study responsiveness to thyrotropin releasing hormone (TRH) in health and disease. More specifically, to study serum TSH response to TRH in patients with Graves' ophthalmopathy. 5) To study the biological significance of T4. To evaluate whether hormonal efficacy of T4 is due only to its conversion, in vivo, to T3 or does it possess intrinsic hormonal activity of its own. 6) To study the relationship between serum levels of thyroid stimulating immunoglobulins and serum T3, T4 and T3/T4 in Graves' disease. 7) To study the pattern of change in circulating thyroid hormones in response to administration of TSH in health and thyroid disease.